Run work asynchronously and compose Futures

Canonical source: protos/examples/concurrency/future-chain.protos at locked revision e99b712ecf658a527b1fe82c8beba2c1286d8dc9.

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// Asynchrony belongs to closure execution: future() schedules this closure.
initial: (() => { 20 }).future()

// then() always returns another Future and automatically flattens a Future
// returned by the transform.
doubled: initial.then((value) => { value * 2 })
answer: doubled.then((value) => {
    (() => { value + 2 }).future()
})

// value() is the ordinary Future observation/suspension operation.
print(answer.value())

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